1. Field of the Invention
The invention relates in general to a RGB signal processor of a scanner, and more particularly, to a circuit of a scanner to perform color space conversion on an RGB signal.
2. Description of the Related Art
The sensor used in the conventional color scanner includes a color charge coupled device (CCD) or a contact image sensor (CIS). The color charge coupled device is composed of several sensor cells to detect the light intensity of the three primary color lights, red light, green light and blue light. According to the detected result, an R charge signal, a B charge signal and a G charge signal are output. A signal process is required for the R, B and G charge signal to become the input signals for the subsequent circuit. FIG. 1 shows a part of the circuit for the signal process of a conventional scanner. The R charge signal, the G charge signal and the B charge signal are sent to the sampling-amplified-offset devices 102, 104 and 106. After sampling, amplifying and level compensations for the R charge signal, the G charge signal and the B charge signal by the sampling-amplified-offset devices 102, 104 and 106, R, G and B analog signals are generated. The multiplexer 108 sends the R, G and B analog signals to the analog-digital (A/D) converter 110. The analog-digital converter 110 then converts the R, G and B analog signals to digital signals and output them to the subsequent circuit.
When the color scanned result is output in gray scale, any one of the R, G and B analog signals is selected as the gray scale analog signal output. As shown in FIG. 1, when the multiplexer 108 selects the R analog signal as the gray scale analog signal output to the analog-digital converter 110, the G and B analog signals are not output to the analog-digital converter 110 via the multiplexer 108.
The above method uses the brightness of the R analog signal to determine the level of gray scale. When the brightness of the R analog signal at one pixel increases, the color of the pixel approaches white. In contrast, the color of the pixel approaches black when the brightness decreases. However, when the brightness of the R analog signal for the pixel is low, this means that the brightness of the G and B analog signals for the pixel is too low. If the brightness of either the G or B analog signal is high, the gray scale level of the displayed color for the pixel is incorrect.
In the RGB color model, the image is composed of three independent images. Each primary color corresponds a plane. When the three image planes are transmitted to the RGB display, a frame of a color image is obtained by combining these three image planes. Therefore, when the image itself is represented by three color planes, it is meaningful to apply the RGB model for the image process. On the other hand, RGB model is applied to most of the color camcorders used to obtain digital images. Therefore, it is a very important model for image process.
Another important model is the yuv model. The advantage for adapting the yuv model is that the brightness y can be separated from the correlated u and v. Further, the RGB color model can be converted into other models such as the theoretical three primary colors stimulated values X, Y and Z, the Adams chrominance-brightness space, and the CYM (cyan magenta yellow) color model. The yuv model using the RGB color model conversion is introduced as an example here.
When using the RGB color model in the yuv color model, the R, G and B analog signals are converted into individual digital signals. A software (such as a conversion program) is used to convert the digital signals into parameters of the yuv color space. The drawback of using the software to convert the RGB color model signal and the yuv color model is extremely high time consumption.